The present invention enables measurement of the mass of a moving object if its composition is known. The invention has application to present experiments in controlled fusion and to future fusion reactors.
The tokamak design fusion reactor maintains a doughnut-shaped high-temperature plasma of hydrogen isotope ions by means of rings of magnets. Replenishing the plasma with hydrogen (which is the fuel of the reactor) is necessary to maintain a sustaining fusion reaction. Also, it is considered preferable to provide hydrogen fuel to the interior of the plasma. Different methods have been considered for this operation of fueling the plasma in the tokamak ring. One highly considered technique consists of injecting frozen pellets of hydrogen isotopes into the plasma. As discussed by S. L. Milora, in "Review of Pellet Fueling", J. Fusion Energy, Vol. 1, page 15 (1981), frozen hydrogen isotope pellets can be made and injected into the plasma at a speed of approximately 10.sup.3 meters per second. This injection technique enables the hydrogen fuel pellets to penetrate deep into the plasma before evaporating, thereby providing the hydrogen fuel to the interior of the plasma.
Further research into the hydrogen pellet injection technique as well as optimizing the operating parameters of an actual fusion reactor will require accurate measurement of the amounts of hydrogen supplied in such pellets. Accurate measurement of the mass of the hydrogen pellets is necessary as a diagnostic for the pellet injector equipment. When functioning properly, the pellet injector should produce pellets within a known range of size. With a reliable, accurate, non-destructive method for measuring pellet mass, it is possible to monitor the performance of the injector.
Accurate measurement of the mass of the hydrogen pellet is also important for studies of pellet ablation and penetration. Pellet ablation rates and penetration into the plasma are a strong function of pellet mass. Comparison between experiments and theoretical models of pellet ablation can only be made if reasonable values for pellet mass are available. Hydrogen pellet mass measurement is also necessary for studies of plasma profile relaxation and plasma transport subsequent to pellet injection. Earlier work has shown that the response of the tokamak plasma to pellet injection can depend strongly on pellet mass. There seems to be a threshold in mass, above which the beneficial effects of central fueling are seen. Further studies will require an accurate determination of pellet size.
Present techniques for manufacturing such pellets do not provide pellets of uniform size. Weighing such pellets is difficult because hydrogen pellets are created and maintained at extremely low temperatures on the order of 10.degree. Kelvin. Also, some of the pellet mass is lost during injection. It is therefore necessary to develop an accurate and nondestructive method for determining the size of the pellets on a shot to shot basis.
Two methods to determine the mass of a moving hydrogen pellet were suggested in an article by P. B. Jensen and V. Anderson, entitled "Handling of Deuterium Pellets for Plasma Refueling" J. Phys. D. Appl. Phys. Vol. 15, page 785 (1982). The first of these methods involves capacitive sensing. The pellet is injected between two capacitor plates. The passing pellet slightly alters the capacitance and this slight alteration can be measured and related to the mass of the pellet. In the second method described by Jensen and Anderson, a microwave cavity is provided through which the pellet can pass. The cavity is mistuned from its resonant frequency so that power is reflected back to the driving amplifier from the cavity. The amplitude of the reflection is measured. Passage of a pellet through the cavity changes the resonant frequency of the circuit which causes a change in the amplitude of power reflected back to the amplifier. This change in amplitude can be measured and related to the mass of the pellet. It can be appreciated that this latter method is an analog method and would rely upon regular and careful calibration of the components for accurate measurements.
Therefore, it is an object of this invention to measure the mass of a moving hydrogen pellet with a method that does not rely on calibration, but instead utilizes a direct theoretical relationship.
It is a further object of the present invention to measure the mass of a moving pellet in an accurate and nondestructive means.
It is a still further object of this invention to provide a means for measuring the amount of hydrogen fuel supplied via pellet injection to the plasma of a fusion reactor having a tokamak ring design.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.